1. Field of the Invention
The present invention relates, generally, to methods for producing silicon carbide and, more particularly, to methods for producing silicon carbide having small particle size. Specifically, the present invention relates to improved methods which are more efficient than previously available for producing silicon carbide particles of very small diameter.
2. Description of the Prior Art
Silicon carbide is an abrasive material which is widely used in a variety of grinding and polishing applications. Its hardness is 9.5 on the Mohs scale and, therefore, is only slightly less than the hardness of diamond (which is 10 on the same scale).
Silicon carbide is conventionally made by reacting silica particles with carbon particles at 1800-2000.degree. C. The chemical reaction for the process (known as the Acheson process ) is: EQU SiO.sub.2 +3C.fwdarw.SiC+2CO
Silicon carbide was first made by this reaction in 1891 in an electric arc furnace. High temperature is required for this reaction because the silica and carbon are contained in separate particles. At 1800.degree. C., silica vaporizes, allowing the two reactants to come in contact.
Another known process involves the reaction of elemental silicon and carbon particles at 1100.degree. C. to make silicon carbide, as follows: EQU Si+C.fwdarw.SiC
Silicon carbide can also be produced with reactions between silanes and a hydrocarbon: EQU SiH.sub.4 +CH.sub.4 .fwdarw.SiC+4H.sub.2 EQU SiCl.sub.4 +CH.sub.4 .fwdarw.SiC+4HCl
Because the Acheson process requires high temperature and a long reaction time, particle growth during the reaction is significant and results in a relatively large particle size distribution (greater than 1 micron) that is not sinterable. These particles must be ground to a smaller size before they can be sintered.
The other two processes discussed above proceed at lower temperatures than the Acheson process, and the reaction times are shorter so that a small particle size distribution (less than 1 micron) results that can be sintered directly. The cost of the silicon sources (i.e., silicon and silane) for these two processes, however, are expensive as compared to the cost of silica, thereby resulting in an expensive silicon carbide product.
Today silicon carbide powders are produced in various grades. The differences between the high and low grades are purity and particle size. Low grade silicon carbide consists of relatively low purity (less than 98%) particles having a diameter of 0.1-10 millimeters. High grade silicon carbide consists of high purity particles (greater than 98%) having a diameter of 0.5 to 5 microns. Low grade silicon carbide is widely used as an abrasive in polishing and grinding wheels.
Because of its high melting point (2200.degree. C.), silicon carbide is also used as refractory in high temperature furnaces. High grade silicon carbide is also sintered to make various types of components, such as fittings, feed throughs, etc. for high temperature applications.
Conventional manufacturing methods for producing silicon carbide utilize electric-resistance furnaces to provide the thermal energy which is necessary to produce the material. Such methods use a significant amount of energy. A solar energy furnace has also been used.
The costs associated with handling large quantities of electricity, and the costs associated with crushing the silicon carbide to smaller particles, account for the major costs in producing the material using conventional techniques.
U.S. Pat. No. 4,419,336 (Kuriakose) describes an improved electric resistance furnace for producing silicon carbide. U.S. Pat. No. 4,534,948 (Baney) describes a process for producing silicon carbide using specific polysilane polymers as starting materials. The polymer is heated to 1600.degree. C. in an inert atmosphere to form silicon carbide. The main advantage of this approach is that the polymer can be pre-formed into fibers or other shapes which the silicon carbide assumes when it forms. These patents do not describe a process for forming silicon carbide of very small particle size.
U.S. Pat. Nos. 4,162,167; 4,789,536; 4,904,622; and 5,021,230 describe manufacture of silicon carbide particles using silica particles and carbon particles which are mixed and then heated to a very high temperature.
U.S. Pat. No. 4,327,066 describes the manufacture of silicon carbide particles by heating silica particles in an atmosphere of hydrocarbon gas and hydrogen gas in a one-step process.
Japanese Patent Application No. 61-6109 describes a process for forming silicon carbide by reducing silica by means of a hydrocarbon at 1300.degree. -1500.degree. C. The silica must be heated to this temperature range before the non-heated hydrocarbon gas is introduced. The hydrocarbon gas cannot be pre-heated or else it is said to decompose before reacting with the silica.
Japanese Patent Application No. 61-6113 describes a process for manufacturing metallic silicon. Silica powder is reacted with a hydrocarbon at a temperature of at least 1300.degree. C. such that part of the silica powder is reduced and converted into silicon carbide. Then silica powder and silicon carbide are charged into a furnace at a temperature of at least 1800.degree. C. to produce metallic silicon.
At the temperatures involved in the one-step process of U.S. Pat. No. 4,327,066 and the foregoing Japanese applications, the rate of pyrolysis of the hydrocarbon is so fast that the carbon will not coat the silica but will form its own distinct particles. This results in distinct silica and carbon particles (similar to the conventional Acheson process). If the carbon does not coat the silica particles, the silica will vaporize as silicon monoxide at the high temperature involved in the process described in the aforementioned U.S. patent and Japanese applications. As a result, there are low product yields.
U.S. Pat. No. 4,869,886 describes a process for producing high density silicon carbide sinters. Particulate silicon is produced by introducing a. silicon compound to a first reaction zone at a temperature higher than the melting point of silicon to form fused spherical silicon particles. These particles are then reacted with a carbon compound at a lower temperature to produce silicon carbide.
U.S. Pat. No. 4,900,531 describes a process in which a silicon-containing precursor gas is heated to yield molten silicon which can then react with carbon walls of the reactor to produce silicon carbide. The resulting product is not a powder.
U.S. Pat. No. 5,082,872 describes a process for producing ceramic materials by pyrolysis of preceramic polysilanes by rendering the polysilanes infusible prior to pyrolysis by exposure to UV radiation in the presence of a reactive gas.
U.S. Pat. No.5,093,039 describes a process for producing electrically conductive sintered silicon carbide by mixing two sizes of silicon carbide particles and then heating the mixture. The small silicon carbide particles are produced by reacting a silicon halide and a hydrocarbon.
There has not heretofore been provided a process for producing silicon carbide particles of small particle size in an efficient manner with high yields.